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Green Roofs and Green Roof Research at the University of Tennessee
By Michael Ross, SITES AP, ASLA, Assistant Professor, Plant Sciences Department + School of Landscape Architecture, University of Tennessee
and James D. Zimmerman, MLA Graduate Student, School of Landscape Architecture, University of Tennessee
Green roofs have been used for centuries to mitigate stormwater, insulate buildings, preserve architecture, and increase sustainability in the built environment. (Dunnett, 2008). Throughout Scandinavia and in the US Great Plains, sod, native grasses and forbs, and earthen roofs have been a traditional building material.
As a design element and constructional component, green roofs are well understood. Loading rates, water retention, architectural modification, and individual components can often be adjusted and optimized to meet specific outcomes. This flexibility allows variability across green roof systems that will be unique to each project. Green roofs help insulate buildings, reduce albedo (light and radiated energy reflectance), mitigate urban heat island effect, capture stormwater, and provide biological resource habitat.
The design-build firm Living Roofs, Inc. in Asheville, NC specializes in green roofs, and their founding partner and design principal Kathryn Ancaya, PLA, describes these installations as:
“…more than a decoration, green roofs are a powerful and effective tool for building regional resilience. Green roofs address significant climate challenges impacting our region, specifically: urban heat, biodiversity loss, and the damaging effects of stormwater runoff. A green roof also presents an opportunity to create multi-species habitat, restore native plant communities, and provide stopover sites for migratory species. Whether we are working on a brownfield site on the edges of Columbia, SC, a multi-story apartment building in downtown Asheville, or a tiny kiosk in a neighborhood park, we look to native plants and ecology for inspiration. Rooftops are an unlikely space for plants to grow and present a unique set of environmental conditions”
(K. Ancaya, personal communication)
There are three main types of green roof systems that allow for opportunities across a variety of conditions: extensive, semi-intensive, and intensive green roofs. For years now, extensive green roofs have done a fair job of meeting the basic green infrastructural needs of the built environment. They are reasonably light, mitigate stormwater, add thermal insulation to a building, and are adaptable enough to have decent survival rates. Sedums are typically the species of choice for extensive green roofs and are light enough that they are often the best option to retrofit an existing roof. Companies have even gotten to the point that they can pre-grow sedum trays that are then placed in sequence on the roof for an efficient and relatively inexpensive green roof. Extensive green roofs are relatively low maintenance once established.
Semi-intensive green roofs are similar to extensive green roofs as they are still lightweight and require relatively little specialized structural support. While this type of green roof still mitigates stormwater and adds thermal insulation, they have an additional ecological function. Instead of using sedums as the plant material, semi-intensive green roofs begin to incorporate low growing, lightweight grasses, forbs, and perennials. Using a diversity of these plant types allows for higher ecological function and ecosystem services by increasing genetic and structural diversity, organismal interactions, and aesthetic performance. The design-build firm Living Roofs Inc. project Camp Mending Heart (Figure 1) is a good example of a diverse semi-intensive green roof.
Intensive green roofs require more structural support, higher initial investment, and more intensive maintenance. However, because they are intensive, these green roofs allow for larger and more complex plantings and typically provide an amenity space. They can even be experienced or programmed to serve as a garden or park. Intensive green roofs are occupiable for human use. The overall soil depth of intensive green roofs can range from 8" – 30", which requires greater structural support. Intensive green roofs tend to be much more expensive to design and install than extensive or semi-intensive roofs, yet tend to provide greater long-term benefits that include energy cost savings, extended roof lifespan, and increased property value.
Some landscape architecture firms, such as Living Roofs Inc., Omni Ecosystems, and Andropogon Associates have embraced green roofs as all or part of their work. Omni Ecosystems’ business model includes design and development of green roofs, such as Omni Ecosystems Headquarters (Figures 2, 3), and green roof products, such as Omni Infinity Material. The Omni Ecosystem Headquarters green roof includes a diversity of native plantings and an ADA accessible rooftop deck. Omni describes their Infinity Media as a “horticulture growing medium that is used as rooting substrate supporting plant growth in onstructure applications” (Omni Ecosystems, 2024).
Andropogon Associates include green roofs in several of their designed projects, such as U.S. Coast Guard Headquarters, INCYTE Corporation, and SUNY College of Environmental Science and Forestry (Figures 4, 5, 6). During the construction of the SUNY green roof, Andropogon’s design team experimented with test plots on an adjacent building’s rooftop (Figures 7, 8). During this experiment, they tested several variables, including soil depth and planting mixture, that helped to determine the best strategy for vegetated areas of the planned green roof. These firms have been developing and researching green roofs in the professional realm for several years and have been the precedent for current green roof research occurring at the University of Tennessee.
As part of the Living Systems Design Group, established within the Ross Lab at the University of Tennessee, green infrastructural applications are deployed and evaluated in terms of biodiversity, species richness, and ecological performance. At present, these applications serve as core elements of experiential learning and sources of scholarly outputs. Ecological systems that perform multiple services in designed and built environments that include green roofs provide interesting to study the impacts of climate change, species competition, succession, colonization, and occupation.
The Green Roof Experimental Lab at the University of Tennessee seeks to explore the ecological function, hybrid ecology, and ecosystem service production of green roofs (Figure 9). This project was made possible through collaboration with the Department of Plant Sciences, School of Landscape Architecture, Department of Earth and Planetary Sciences, and the UT Gardens. Experimental research is conducted on biodiversity and spontaneous plant colonization, mycorrhizal interactions, organic decomposition rates, organismal interactions, and aesthetic performance of roof systems that preference native plant communities. Green roof media was sourced from Omni Ecosystems.
Units were designed and constructed by students in the College of Architecture and Design and Department of Plant Sciences Sustainable Landscape Design concentration (Figure 10). Students were trained in Omni Ecosystem’s methodologies and green roof media establishment protocols. Units were designed to be mobile in case of emergency by skid-steer. The units aim to provide immersive teaching opportunities, a method for reducing ambient temperatures in built systems, a means of reducing stormwater runoff, create novel native habitats, improve air quality, reduce ambient noise, and to provide research opportunities (Figure 11). Green roof research is also in progress on the roof of the Sustainable Landscape Design concentration’s storage unit, where a native pocket prairie is being established. In 2009, a sedum green roof was installed.
Over the last couple of years, a decision was made to retrofit the storage shed roof with an ecologically diverse, functional pocket prairie. Old media was removed, and Omni Ecosystems media was incorporated to develop a hybrid system. Native species were interplanted into the existing sedum, creating a hybrid prairie and sedum system with approximately 17 different species (Figures 12, 13).
In 2024, the new Agriculture and Natural Resources (ANR) building on the University of Tennessee Institute of Agriculture’s campus will be completed. An intensive green roof is proposed to be built where additional research and design opportunities will continue to develop. As proposed, this green roof installation will support several different and ongoing program initiatives, including monitoring the performance of green roofs across time. The design proposal will contain an outdoor classroom, a multi-use plaza, green roof trial beds, and a weather and green roof monitoring station.
Green Roof research initiatives at the University of Tennessee stand at the forefront of advancing sustainable solutions for the built environment. Through collaborative efforts across disciplines and partnerships, with industry leaders like Omni Ecosystems, students and faculty are not only exploring the ecological functions and ecosystem services of green roofs, but also actively contributing to their evolution and innovation. The future of green roof research at the University of Tennessee will further enhance our understanding and application of green infrastructure for a more sustainable and resilient future.
References and Additional Resources
Ancaya, K. (2024, March 7). Kathryn Ancaya quote.
Andoropogon Associates. (n.d.). SUNY College of Environmental Science and Forestry. Andropogon. https://www.andropogon.com/project/suny-college-of-environmental-science-forestry/
Andropogon Associates. (n.d.). Incyte Corporation. Andropogon. https://www.andropogon.com/project/incyte-corporation/
Andropogon Associates. (n.d.-b). U.S. Coast Guard Headquarters. Andropogon. https://www.andropogon.com/project/u-s-coast-guard-headquarters/
Archtoolbox. (2021, September 19). Green roof systems: Intensive, semi-intensive, and extensive. https://www.archtoolbox.com/green-roof-systems
Environmental Protection Agency. (2023). Using Green Roofs to Reduce Heat Islands. EPA. https://www.epa.gov/heatislands/using-green-roofs-reduce-heat-islands#:~:text=Green%20roofs%20provide%20shade%2C%20remove,effect%2C%20particularly%20during%20the%20day
Green Roof Technology. (2021, October 16). Intensive green roof. https://greenrooftechnology.com/green-roof-finder/intensive-green-roof/
Living Roofs, Inc. (n.d.). Camp mending heart: Single family :: Living roofs inc.. Camp Mending Heart | Single Family :: Living Roofs Inc. https://www.livingroofsinc.com/projects/camp-mending-heart
Omni Ecosystems. (n.d.). Omni Ecosystems Headquarters. https://www.omniecosystems.com/portfolio/omni-ecosystems-headquarters
Omni Ecosystems. (n.d.-b). Omni Rewild. https://www.omniecosystems.com/omni-rewild
SLD Living Systems Design [Ross] Lab https://sldlivingsystemslab.utk.edu
